1. Field of the Invention
The present invention relates to an imaging device that forms an image of an photographic subject by using a solid-state imaging element such as a CCD or the like, and particularly relates to an imaging device that is provided with a micro-lens on a light receiving surface of the solid-state imaging element.
2. Description of the Prior Art
A solid-state imaging element such as a CCD or the like (hereinafter termed an imaging element) is becoming more miniaturized in recent years and also the number of picture elements of the imaging element is increasing. As a result, an area of a light receiving section of each photo-electric element must be small. If the area of the light receiving section becomes small, the sensitivity drops. An imaging element is known in which a micro-lens is formed as a unitary body in front of each light receiving section as shown in FIG. 20 and a light flux from a photographic subject is converged to the light receiving section so that reduction of the sensitivity is compensated.
However, if the micro-lens that has a certain curvature is attached, the sensitivity of the imaging element varies according to a value of an aperture that is positioned in front of the imaging element, that is on the photographic subject side, or a distance between an exit pupil position of a photographic lens and the imaging element. For example, since the light flux from the photographic subject comes into the micro-lens almost in parallel if the aperture value is great, most of the light flux is received on the light receiving section and the sensitivity becomes good. In the same way, since the light flux from the photographic subject comes into the micro-lens almost in parallel if the distance between the exit pupil position and the imaging element is long, the sensitivity becomes good. On the other hand, if the aperture value is small or the distance between the exit pupil position and the imaging element is short, a light flux from the photographic subject that obliquely comes into the micro lens increases. Accordingly, the light flux is refracted greatly by the micro-lens according to the incident angle of the light flux and some of the light flux is not received by the light receiving section. As a result, the sensitivity drops.
An imaging device is known in which dropping of the sensitivity is compensated by electrically amplifying image data outputted from the imaging element with an amplification factor according to the aperture value to solve the above-mentioned problem (Japanese Laid-Open Patent Application No. 6-178198). However, since the output of the imaging element includes noise, if the amplification factor is set to a great value to compensate the drop in sensitivity, the noise is also amplified and the picture quality becomes low. The low picture quality like this is not a substantial problem for a video camera or the like by which a dynamic image is recorded and reproduced. But the low picture quality is a substantial problem for an electronic still camera or the like which handles a still image, because the low picture quality is conspicuous even if the picture quality is not so low.
Japanese Laid-Open Patent Application No. 6-311422 discloses an imaging device in which a shutter speed is modified according to the aperture value or the distance between the exit pupil position and the imaging element. However, if the shutter speed is different from one that a photographer recognizes, the photographer feels strange and a camera vibration by hand likely occurs. For example, if a photography is performed with a lower shutter speed than 1/60 sec and an open aperture, the sensitivity of the imaging element drops to about 50%. The shutter speed must be set low as the photographer feels the low shutter speed in order to compensate this dropping of the sensitivity. As a result, the photographer feels strange.